The present invention relates to a control apparatus for an internal combustion engine, which is provided with a variable valve actuation mechanism for changing the valve properties of engine valves. The present invention also pertains to a method for controlling an internal combustion engine.
Usually, in starting a vehicle, after the rotational speed of an engine has increased to some extent, a friction clutch is engaged to start power transmission from an engine output shaft to an axle. In starting a vehicle provided with a liquid clutch such as a torque converter, likewise, power transmission to an axle is started after the rotational speed of an engine has increased to some extent. Therefore, depending on the mode of power transmission, for example, as shown in FIG. 18, when power transmission is started, that is, when a rotational speed NT of a clutch output shaft starts to increase (time t31), the increase rate of an engine rotational speed NE stagnates temporarily, or decreases temporarily in some cases.
Some internal combustion engines mounted on vehicles are provided with a variable valve actuation mechanism for appropriately changing the valve properties such as valve timing and lift of an intake valve and an exhaust valve. The variable valve actuation mechanism regulates, for example, an overlap period (valve overlap amount) of the valve opening period of the intake valve and the valve opening period of the exhaust valve, and hence executes what is called exhaust gas recirculation (internal EGR). The regulation control of valve overlap amount has often been carried out in recent years because it decreases a pumping loss and improves exhaust emission.
In this regulation control, usually, the valve overlap amount is changed so as to be increased gradually in starting the vehicle. This regulation control increases an internal EGR amount and hence deteriorates the combustion state, so that it may promote the above-described temporary stagnation of increase in engine rotational speed.
Accordingly, there has conventionally been proposed an apparatus for restricting the valve overlap amount at the time of vehicle start to an amount smaller than the amount matching the engine operating state, for example, as disclosed in Japanese Laid-Open Patent Publication No. 8-277729.
As a prior art document relating to the present invention, Japanese Laid-Open Patent Publication No. 10-318001, described below, can be cited in addition to the above-described Japanese Laid-Open Patent Publication No. 8-277729.
According to this conventional apparatus, the increase in internal EGR amount at the time of vehicle start is restrained, by which the temporary stagnation of increase in engine rotational speed can be restrained. However, even such an apparatus may have a drawback as described below.
For example, as a variable valve actuation mechanism, a variable valve timing mechanism that operates according to the pressure of supplied oil to variably set the valve timing has been known. Also, a variable valve timing mechanism provided with a lock mechanism that is operated by the pressure of supplied oil to lock the operation of variable valve timing mechanism so that the valve timing is a predetermined timing has been known.
In the variable valve timing mechanism provided with the lock mechanism, if the oil pressure is changed to change the valve timing in a locked state, first, the lock is released by the oil pressure, and then the change of valve timing is started. Therefore, if the above-described restriction of change of valve overlap amount is executed, the change of pressure of oil supplied to the lock mechanism is also restricted, which may retard or disable unlocking. This unfavorably becomes one cause for hindering a rapid change of valve overlap amount when the valve overlap amount is changed to the amount matching the engine operating state after the above-described restriction of change has been lifted.
On the other hand, an internal combustion engine provided with a variable valve actuation mechanism for each intake valve and exhaust valve has been known. In such an internal combustion engine, because of its characteristics, a difference in convergence speed of actual valve properties to desired properties arises inevitably between the variable valve actuation mechanisms when the valve properties are changed by each variable valve actuation mechanism.
When the convergence speed is changed in various modes, the change in intake air charge efficiency at the transition time when the valve properties change differs, and some change may decrease the increase rate of engine torque. This unfavorably becomes one cause for promoting the temporary stagnation of increase in engine rotational speed.
Thus, a decrease in controllability caused by vehicle acceleration cannot be avoided even by the above-described conventional apparatus, and there is yet room for improvement. The above-described drawback occurs not only at the time of vehicle start but also at the time when the vehicle is accelerated from a relatively low running speed.